1. Field of the Invention
The present invention relates to a plasma cutting apparatus for cutting products from a plate material and to a control unit thereof.
2. Description of the Related Art
A plasma cutting apparatus for conducting plasma arc cutting by supplying a rotating flow of an assist gas into the zone around a rotating plasma arc, the flow rotating in the same direction as the arc, is known as a plasma cutting apparatus of this type. In plasma arc cutting, the cross section of the plate material assumes a tapered shape which narrows downward due to a plasma arc contraction, but in such a plasma cutting apparatus, the bevel angle formed by the cutting surface and the surface perpendicular to the lower surface of the plate material (in other words, the degree of tapering of the cutting surface) can be adjusted by adjusting the assist gas rotation intensity (or assist gas flow rate).
However, the degree of tapering of the cutting surface usually differs depending on cutting speed. For this reason, for example, even if the rotation intensity of the assist gas (or assist gas flow rate) is set so that the bevel angle is exactly zero degrees, when the cutting speed changes, the bevel angle is overcorrected and does not become zero degrees.
Furthermore, as shown in FIG. 1, a phenomenon of cutting delay at the lower side with respect to the upper side is observed at a cutting frontmost surface 110 of a plate material 300 (a distance 130 corresponding to the delay of cutting at the lower edge with respect to the upper edge of the cutting frontmost surface will be hereinbelow referred to as “cutting delay”). For this reason, in particular, in locations with significant changes in cutting direction, as in the corners and sharp curves, a positional shift occurs between a trajectory 150 on the upper side and a trajectory 170 on the lower side of a cutting groove 50.
In order to avoid the degradation of cutting quality caused by the aforementioned effects, a plasma cutting method was suggested in which an advantageous bevel angle is obtained by adjusting the assist gas flow rate (or assist gas rotation intensity) according to the cutting speed, more specifically, by making the assist gas flow rate (or assist gas rotation intensity) comparatively high when the cutting speed is comparatively high and making the assist gas flow rate (or assist gas rotation intensity) comparatively low when the cutting speed is comparatively low (for example, Japanese Patent Application Laid-open No. 2000-317639).
However, even when plasma arc cutting is conducted by the aforementioned method representing conventional technology, when a hole is cut in the product, the cutting quality is not necessarily good. This is due to the reasons (1) to (4) as follows.
(1) Cutting Delay
Because the hole size is extremely small by comparison to that of the product contour, when the hole is cut, the effect of cutting delay is stronger than when the contour is cut. For example, as shown in FIG. 2, if cutting delay occurs in hole cutting, the diameter DL on the lower side becomes less than the diameter DU on the upper side of the hole 310 and this is readily visible.
(2) Flow of Plasma Arc in the Center Direction Caused by Gas Flow
In contour cutting, no space appears on the left and right sides of plasma arc with respect to the plasma arc movement direction during cutting (in other words, no space appears on the left and right sides of the cutting groove). For this reason, almost equal pressure acts from the right and left sides on the plasma arc. However, in hole cutting, when the hole diameter is small with respect to a cutting groove width formed by plasma arc cutting, a space is sometimes formed on the right or left side (in other words, on the hole center side) of the plasma arc during cutting. For this reason, the pressure pushing the plasma arc toward the hole center sometimes becomes higher than the pressure pushing the plasma arc to the outside of the hole. As a result, during hole cutting, the plasma arc flows toward the hole center, which results in different trajectories on the upper side and lower side of the hole and a state, for example such as shown in FIG. 2, is assumed.
(3) Peaks and Valleys Appearing on the Cutting Surface
In most cases when hole cutting is conducted, as shown in FIG. 3, the hole 310 is formed and cut by piercing a prescribed location P1 on the inner side of the hole 310 with a plasma arc, then forming a plasma arc, moving the plasma arc into the prescribed location (referred to as “circling start position”) PS on the outline of the hole 310, and then rotating the plasma arc along the outline of the hole 310 from the circling start position PS (the dot line shows the curve trajectory). In this process, the cutting frontmost surface formed by the plasma arc is curved and because of the above-described phenomena, such as cutting delay and plasma arc flow toward the center, even if the plasma arc is rotated with good accuracy along the outline of the hole 310, a certain displacement occurs between the circling start position PS and circling end position PG of the plasma arc, that is, the location of the plasma arc at the moment the hole 310 was formed, the cutting frontmost surface produced by plasma arc is produced, but is not curved, and therefore peaks and valleys 320 appear on the cutting surface.
(4) Degradation of the Nozzle of the Plasma Torch Ejecting the Plasma Arc
As the number of cutting cycles increases, the nozzle of the plasma torch for spraying the plasma arc is degraded. As a result, the pressure balance on the left and right sides with respect to the plasma arc sometimes becomes different. In hole cutting, as was described in section (2) hereinabove, this pressure balance affects cutting quality to a greater extent than in contour cutting. For this reason, if the nozzle is degraded with the increase in the number of cutting cycles, the hole cutting quality may be also degraded.
Among the above-described factors (1) to (4), at least the cutting delay, which is the factor (1), is practically unimportant in terms of hole cutting quality, but adversely affects cutting quality of the product contour.